This invention pertains to a method for applying complex patterns in close alignment with each other on both sides of a single high temperature superconducting (HTS) wafer.
In common practice in the art, a HTS thin film is deposited upon one or both sides of a thin substrate wafer. For most applications today, only one device is fabricated on a single wafer; that is, only one side of the wafer receives a pattern. The reverse side of the wafer, if employed at all, is employed as a heat sink or other structure that has no strongly preferred geometric orientation with respect to the pattern on the first side.
There is increasing interest in applications wherein it is desirable to couple two superconducting electronic devices electromagnetically at microwave or radio frequencies. Such applications include microwave power transmission, bandwidth and bandpass filters, and imaging, such as magnetic resonance imaging (MRI). In such applications, there is a considerable benefit to laying down the devices to be coupled on opposite sides of the same wafer. In addition, the patterns on the two sides often need to be aligned to micron scale in order to achieve optimum performance.
While the substrates commonly employed in the art, such as LaAlO3, sapphire, MgO, NdGaO3 and yttria stablized zirconia are themselves transparent at visible wavelengths, the HTS thin films, such as Tl2Ba2CaCu2O8 or YBa2Cu3O7xe2x88x92x are opaque at all practical wavelengths. The result is that the first patterned side is not visible while applying the pattern to the opposite side, which makes registration of the two patterns extremely difficult. Two-sided patterned waters are disclosed in Shen, U.S. Pat. No. 5,750,473. Until now, the best known method in the art for fabricating such a device is represented by the model MA6 Mask Aligner manufactured by Karl Suss K G GmbH and Co., Munich, Germany. The MA6 is a complicated apparatus equipped with both bottom side and top side microscopes and image storage capabilities employed to permit simultaneous viewing of the two sides of the opaque wafer. The major drawback for an apparatus such as the MA6 is that no matter what equipment configuration is employed, there will be some device geometry which requires placement of reference marks which will be inaccessible to the optical configuration in place in any device such as the MA6. In most instances, the optical configuration could be changed to accommodate the new sample geometry, but this is an expensive and time consuming undertaking at best. Thus, the art does not provide a flexible means to achieve double-sided patterning of HTS wafers of arbitrary geometry aligned with micron scale precision.
The present invention provides for a method for providing alignment reference marks on opposing sides of a double-sided high temperature superconducting wafer comprising the steps of:
applying at least one reference mark to a first high temperature superconducting thin film on a first side of a transparent wafer substrate;
patterning a second high temperature superconducting thin film on a second side of the wafer to provide at least one window aperture through which said at least one reference mark is visible.
FIGS. 1-4 provide a schematic illustration of the method of the invention, in which:
FIG. 1 shows the patterned first side of an HTS wafer with reference marks;
FIG. 2 shows the unpatterned second side after the etching of windows therein at the approximate location of the first side reference marks;
FIG. 3 shows the initial positioning of the photomask carrying the pattern to be applied to the second side of the wafer, the photomask having reference marks not fully aligned with those on the first side; and
FIG. 4 shows the final positioning of the photomask after precision of alignment of the photomask reference marks with the first side reference marks visible through the second side windows.
FIGS. 5a-5c depict respectively the first side pattern, the window apertures and the second side patterns for use in a magnetic resonance imaging device.
FIGS. 6a-6c depict respectively the first side pattern, the window apertures and the second side patterns for use in a nuclear magnetic resonance spectrometer.
FIGS. 7a-7c depict respectively the first side pattern, the window apertures and the second side patterns for use in a microwave transmission power filter.